1. Field of the Invention
The present invention relates to a dual band antenna for a radio terminal capable of efficient operation at two different frequency bands.
2. Description of the Related Art
In general, to implement a dual band antenna using a single antenna, an additional element such as a choke is required for enabling respective parts of the antenna to independently operate at different frequencies. U.S. Pat. Nos. 3,139,620 and 4,509,056 disclose an antenna employing a choke, to permit operation at multiple frequencies.
U.S. Pat. No. 4,509,056 (the '056 patent) discloses a multi-frequency antenna employing tuned sleeve chokes. FIG. 1 of the '056 patent illustrates a cross sectional view of a monopole antenna operating at dual frequencies. This antenna is suitable for a radio terminal in which the frequency is not isolated by harmonics and the frequency ratio is greater than 1.25. As illustrated, the antenna is composed of a common monopole antenna, a coaxial transmission line having an open end, a shorted end, and a ground plane.
In FIG. 1, a coaxial transmission line choke 12i is formed at the middle of the antenna and has an electrical length .lambda./4 at the higher frequency band of the dual frequency band. At the higher frequency band, the .lambda./4 sleeve choke 12i forms a very high impedance between the open end and an extension element 100 of the coaxial feed line, thereby preventing coupling therebetween. Accordingly, at the higher frequency band, only the portion represented by l functions as the antenna as illustrated in FIG. 1. However, at the lower frequency band, the sleeve choke 12i does not serve as an isolation element so that the entire portion represented by P functions as a monopole antenna.
A drawback associated with the conventional dual band antenna employing a choke is that it is both complicated and large, as compared with a single band antenna. Further, the large antenna may be easily damaged by a trivial impact. In addition, the conventional fixed (i.e., irretractable) antenna may inconvenience a user in carrying the radio terminal.